Pyloric Mechanisms

Abstract

When the stomatogastric nervous system is dissected out of a lobster, pinned out in a dish and perfused with physiological saline, a highly ordered rhythmic pattern of bursts repeating at a rate of one to two cycles per second can be recorded from the pyloric motoneurons. The first time one sees these nerve recordings on an oscilloscope screen, or hears the pattern on an audio monitor, a sense of amazement is experienced: the pyloric pattern is spontaneously generated information. The neurons in that dish are an incredibly sophisticated biological “function generator”, and the immediate question is how does it all work? Could one ever hope to understand and explain the operation of this neural system at a mechanistic, cause-and-effect level? The purpose of this chapter is to attempt just such an explanation. First, as a basis for this discussion, the characteristics and variability of the pyloric burst pattern will be examined. In the following three sections, a review of pyloric mechanisms will be organized around three specific question: (1) Why do the pyloric cells fire in bursts of activity? (2) How are these bursts of activity coordinated to occur with the observed phase relationships? (3) What mechanisms determine the overall frequency of the motor pattern? Finally, we will integrate what we have learned about the circuitry, cellular and synaptic properties into a qualitative “gestalt” picture of pyloric pattern generation. A more quantitative description of pattern generation will be presented in Chapter 7.